When synthesizing Acetophenetidin (Phenacetin) in an Organic Laboratory, you might end up with a cake of crude product filled with impurities from the synthesis process. A giveaway sign of this is a calculated percent yield in excess of one hundred percent. This is not high-quality product. It must be recrystallized in order to refine the product for larger crystals and purer yield.
In order to recrystallize a product for better purity, a solvent must be found that will solvate everything at high temperatures, but will precipitate the pure product at cooler temperatures while keeping the impurities solvated and out of the crystal structure of the reforming product.
Perhaps you are given three options: deionized water, ethanol, and hexane. To test which would be the better solvent, add a bit of the crude product to small amounts of the potential solvents in test tubes. Observe solubility. Then heat the solvents in a boiling hot-water bath and observe solubility at that state. Finish the mock recrystallization by removing the test tubes from the bath and letting them cool to room temperature before sticking them in an ice bath. Observe solubility once again.
The better solvent will preferably not solvate the acetophenetidin at room temperature. This means that the recrystallization will begin earlier in the cooling phase. Everything must be dissolved at boiling. A solvent with a low boiling point would not help here, as it will evaporate away leaving everything as an impure solid stuck to the sides of the test tube. When cooled in ice, crystals must form for the solvent to be worth anything.
Here is some sample data:
- Room Temperature – Crude product appeared to be insoluble
- Boiling – Completely solvated the crude product
- Freezing – Crystals reappeared
- Room Temperature – Completely solvated
- Boiling – Still solvated
- Freezing – Remained solvated
- Room Temperature – Insoluble
- Boiling – Solvent evaporated
- Freezing – The solvent evaporated away in the previous step
The hexane is out as a solvent; its boiling point is too low. Ethanol does not work because the acetophenetidin never precipitates out of it. This leaves only water, and the observations associated with it prove it to be a useful solvent.
It is important to know what to look for when observing the Freezing solubility. Even though the impure cake may enter the test tube as a lump, it will not precipitate that way. Once the test tubes are sitting in the ice bath, let them sit undisturbed for five minutes. Then remove and observe. Wipe the condensation off the tubes and look very closely at them. From farther away, the water and ethanol tubes look much the same. Upon closer inspection and perhaps a swirl, one can see very tiny white particles floating in the water that are absent in the ethanol. That is the pure acetophenetidin and a good sign. These particles may be even smaller than any dust in the tube; close observation is important here.